In the fluid catalytic cracking process for converting high boiling hydrocarbons to lighter hydrocarbons, catalyst particles in a fluidized state are contacted with hydrocarbon in at least one contacting zone. The manner of contacting of hydrocarbon and catalyst can have a marked influence on the performance of a fluid catalytic cracker (“FCC”). It would be advantageous if catalyst particles in the riser of a FCC could be completely and instantaneously mixed with hydrocarbon in a feed mixing zone. However, this is not physically possible. There have been many efforts to optimize mixing of hydrocarbon and catalyst in order to improve the performance of the FCC.
Efforts to improve mixing in the feed mixing zone have focused on the injection nozzles used to inject feed and on the manner in which catalyst and feed are mixed. One purpose of the injection nozzle is to make as fine a dispersal of feed as possible. The process of making fine droplets is known as fluid atomization and is influenced by factors such as orifice size, pressure drop and fluid density and viscosity.
One manner of influencing the mixing feed and catalyst focuses on feed preparation. For example the feed may be mixed with a gas stream prior to atomization. The feed may be discharged through an orifice into a mixing tube prior to contacting catalyst. The feed, the catalyst or both may also be accelerated prior to mixing. A lift gas may be used to accelerate catalyst particles in the riser.
The orientation of feed injectors may also affect feed mixing. In one case, the feed is radially directed through nozzles arrayed around the circumference of the riser. Other arrangements of nozzles and methods of injecting focus on the creation of a venturi effect. This is accomplished by creating a restriction in the flow path of the catalyst, feed or both. Another method and apparatus involves a feed injection arrangement in which feed is injected transversely from the sides of a restricted opening involving different cross-sections in the mixing zone.
There is an advantage to optimizing the mixing zone in a FCC unit without reliance on accelerating the catalyst as this energy input results in a pressure drop and limits catalyst circulation.